The present invention relates generally to gas sensors, and, particularly, to photoacoustic gas sensors.
The following information is provided to assist the reader to understand the invention disclosed below and the environment in which it will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the present invention or the background of the present invention. The disclosure of all references cited herein are incorporated by reference.
The use of diffusive gas sensors to detect the concentration level of gaseous species of interest using the photoacoustic effect is well known. For example, U.S. Pat. No. 4,740,086 discloses the use of a diffusive photoacoustic gas sensor to convert the optical energy of an amplitude modulated light source into acoustic energy when the light mechanically and thermally excites the gaseous species of interest as it diffuses into a sensing chamber upon which the light is incident. Sound waves of an intensity corresponding to the concentration level of the gas within the chamber are generated as the light radiation absorbed by the gas creates pressure fluctuations of a magnitude proportional to the number of gas molecules located within the sensing chamber. These sound/pressure waves are detected by an acoustic detector such as a microphone.
Photoacoustic gas sensors can have mechanical valves to let in the sample gas when open, which then close to trap the sample gas and block external acoustical noise. Valves have the disadvantages of requiring energy to operate and having moving parts which wear leading to limited lifetimes (typical 0.5 to 3 years). A gas diffusion element such as described in U.S. Pat. No. 4,740,086, could be used to simultaneously allow gas diffusion and attenuate external acoustical noise. But the degree of attenuation and the rate of gas diffusion is a compromise. A faster gas diffusion rate typically accompanies reduced external noise attenuation. Thus a photoacoustic sensor that uses a diffusion element (as opposed to valves) is more susceptible to external acoustical noise from the environment entering through the diffusion element.
The output signal of a diffusive photoacoustic sensor is susceptible to noise created by interference from outside sources of air pressure fluctuations, such as wind, vibration and acoustic phenomena. To eliminate such noise, one may incorporate some means of attenuating extraneously generated pressure waves, while attempting to allow the gas to freely diffuse into the sensing chamber for detection. For example, porous members through which gas relatively readily diffuses, but which attenuate the effect of external pressure fluctuations, are often placed at the entrance of photoacoustic sensors. However, one must balance this attenuating effect with a corresponding increase in response time. In that regard, introduction of a sound/pressure attenuating element or elements to reduce noise typically results in a corresponding loss of responsiveness to changing analyte concentration. The specifications for combustible gas detectors of the Instrument Society of America (ISA) require gas concentration level measurement stability at wind speeds of up to 5 meters per second (m/s) with a corresponding response time (to 60% of full scale indication) of less than 12 seconds.
U.S. Pat. No. 7,034,943 discloses a sound/pressure damping element (SDE) designed for use in a diffusive, non-resonant photoacoustic gas sensor (detector). The SDE reduces external, low-frequency noise to acceptable levels while permitting the photoacoustic detector (sensor) to maintain an adequate response time to changing gas concentration levels. In general, the photoacoustic detector of U.S. Pat. No. 7,043,943 includes a first volume having a sensor system for photoacoustic detection therein. The first volume is in fluid connection with the environment through an opening so that the gas analyte can diffuse into the first volume through the opening. The photoacoustic detector of U.S. Pat. No. 7,043,943 further includes a second volume (an SDE volume) in connection with the first volume such that pressure readily equalizes between the first volume and the second volume and such that diffusion of analyte gas from the first volume to the second volume is hindered (or slowed as compared to diffusion of analyte gas into the first volume from the environment). Typically, the second volume is substantially larger (for example, 300 mL) than the first volume (for example, 1 mL) to enhance attenuation of external pressure fluctuations. The first volume can, for example, be connected to the second volume by a channel that is shaped to limit diffusion of analyte gas therethrough. The channel may, for example, be elongated and of small cross section compared to the opening into the first volume. Disadvantages of an SDE include the large gas volume required (especially for explosive gases), the time for the sample gas to fill the large SDE, and the physical size/bulk of the SDE.
U.S. Pat. No. 7,106,445 discloses a photoacoustic gas sensor utilizing diffusion having a sensing volume and an acoustic pressure sensor volume containing an acoustic pressure sensor such that the fluid connection between the sensing volume and the acoustic pressure sensor volume restricts the flow of analyte gas therethrough but does not restrict the transmission of the photoacoustic signal therethrough. The methods and devices of U.S. Pat. No. 7,106,445 provide for controlling the diffusion of the analyte gas within a diffusive photoacoustic gas sensor to improve the response time of the sensor as, for example, compared to a sensor with a large SDE.
U.S. Pat. No. 6,006,585 discloses a photoacoustic (or optoacoustic) gas sensor including a sensor body, a light source, a measurement cell with a gas-permeable membrane, a measurement microphone, and an optical measurement filter between the light source and the measurement cell. The sensor also includes a reference cell that is separate from the measurement cell. The reference cell is generally identical in volume and form to the measurement cell. The reference cell and the measurement cell each include a gas inlet. The reference cell has a reference microphone that is shielded against photoacoustic signals from the gas to be detected. In that regard, the reference cell is substantially free from intensity-modulated optical radiation having an absorption wavelength of the gas to be detected. The measurement signal, which indicates gas concentration, is obtained by subtraction of the signals from the measurement and reference microphones. As a result of the subtraction, interference signals caused by vibrations or air pressure fluctuation are purportedly eliminated (the former through use of the reference microphone which receives no photoacoustic signals from the gas to be measured, and the latter by virtue of the spatially separate reference cell with the reference microphone). See col. 1, line 49 to col. 2, line 7.
As discussed briefly above, photoacoustic sensors can also be sensitive to vibrational energy in their environment of operation. U.S. Pat. No. 4,818,882, for example, discloses the use of two opposing microphones for reducing sensitivity to vibration. The microphones are turned oppositely and are symmetrically positioned about the center of gravity of the air contained in the measuring chamber. The signals of the two opposing microphones are summed to reduce the effect of vibration on the sensor output. Uniform microphones are required for simultaneous compensation of the vibration signal from both the air and the membrane.
Although advances have been made in the field of photoacoustic sensors, it remains desirable to develop improved photoacoustic gas sensors, devices for use in photoacoustic gas sensors and methods of fabricating photoacoustic gas sensors.